Embodiments of the present invention relate to a crosshead-piston rod assembly, specifically in a reciprocating compressor.
In the technical field of reciprocating machines (i.e. machines having a piston moving inside a cylinder such as, for example, internal combustion engines) the piston is usually connected to a crankshaft directly via a connecting rod. Specifically, the connecting rod is linked to the crankshaft via a crankpin. In this arrangement, however, the sideways forces from the connecting rod are transmitted directly to the piston, as the crankpin (and thus the direction the force is applied) moves from side to side with the rotary motion of the crankshaft.
These transverse forces are tolerable in a smaller reciprocating machine. However, in a larger one, such as for example a large reciprocating compressor, this would result in much greater forces, consequently causing an intolerable degree of wear on the piston and cylinder, as well as increasing overall friction.
Therefore, it becomes necessary to decouple the transverse movement of the crankpin from the axial movement of the piston. To perform this function, a crosshead links a piston rod (which is part of the piston) with a connecting rod (which is linked to the crankshaft via the crankpin). The sideways forces transmitted by the connecting rod are absorbed by the crosshead itself, thus allowing the piston rod to move along its axis with negligible transverse load.
A crosshead-piston rod assembly is known in the state of the art, comprising a piston and a crosshead. The piston comprises a head, which is inserted into a cylinder, and a rod attached to the head. The crosshead comprises a main body having a first end linked to the crankshaft and a second end attached to the piston rod. Additionally, the crosshead is provided with bearings attached to the main body and slidably engaged on an internal wall of the cylinder. The bearings themselves absorb the above mentioned sideway forces, allowing the crosshead to maintain the alignment with respect to the longitudinal axis of the piston.
In greater detail, the known crosshead comprises a flange which is attached to the main body, specifically onto the second end. The rod has a shoulder and a neck part. The flange has a through bore into which the rod, specifically the neck part, can fit, while the shoulder of the rod is placed in contact with a surface of the flange. In other words, the rod is partially inserted into the bore of the flange, so that a threaded end emerges from the other side.
To assemble the piston to the crosshead, a nut is screwed onto the threaded end so that the flange is locked between the nut and the shoulder of the rod. The flange is then bolted to the main body of the crosshead. During normal operations of the piston, the loads are mainly of the axial kind. Indeed, this causes a concentration of stress in the zone of the rod between the neck and the shoulder.
The main disadvantage of the known crosshead-piston rod assembly is that such a concentration of stress in a very small area is often enough to bring the material of the rod to plastic deformation, thus compromising the structural integrity of the rod.